Why am I seeing “read-only volume” or “insecure update” errors when I install or update Craft Manager on macOS Sierra?

If you’re on macOS Sierra, you may have encountered the below error messages when attempting to install or update Craft Manager:

This error occurred due to a change in how apps are installed on macOS Sierra. We’ve recently released an update to Craft Manager to address this moving forward. However, those impacted by this problem will need to do a clean install of Craft Manager through the following steps: